Fast fault notifications of an optical network

ABSTRACT

A method and apparatus for fault notification in an optical network are described herein. In one embodiment, an exemplary process includes detecting at a node that at least a portion of a first unidirectional path of an optical circuit is down, where the first unidirectional path is originated from a first terminating node. In response to the detection, the node signals the first terminating node by removing at least a portion of light of a second unidirectional path in an opposite direction of the first unidirectional path of the optical circuit, to indicate a path between the node and the first terminating node is down. Other methods and apparatuses are also described.

This application is a continuation of U.S. patent application Ser. No.10/785,597, filed Feb. 23, 2004 now U.S. Pat. No. 7,499,646, entitled“Fast Fault Notification Of An Optical Network”, now issued as U.S. Pat.No. 7,499,646, and assigned to a common assignee of the presentapplication. This application is also related to the issued U.S. Pat.No. 7,474,850, filed Feb. 16, 2005, entitled “Reroutable ProtectionSchemes Of An Optical Network” and assigned to a common assignee of thepresent application.

FIELD OF THE INVENTION

The present invention relates generally to the field of networking. Moreparticularly, this invention relates to optical networking.

BACKGROUND OF THE INVENTION

An optical network has long enjoyed the sub-60 ms self-healing ringarchitecture. As the network grows, the ring topology is no longersuitable of its cumbersome provisioning and complex ring inter-workingin a large network. The optical mesh network helps solve some of theseissues. However, it suffers from a historically slow fault recoverytime. The ring network is able to achieve sub-60 ms protection timebecause the fault detection and protection switching are performedlocally where the fault occurred. In a mesh network, the fault recoveryprocedure is executed at the source and destination for end-to-end pathprotection. As a result, the fault notification time has contributed toslow recovery time for a mesh network.

Most optical transport networks today are based on electronic switchingequipment which takes light as an input and converts the light intoelectronic data. It then processes the electronic data and converts themback to light. In order to carry the data, different types of framingprotocol have been developed, such as, SONET (synchronous opticalnetwork), SDH (synchronous digital hierarchy), and OTN (opticaltransport network). The framing protocol uses a relatively small amountof bandwidth for its overhead data to carry framing information, errorchecking and monitoring, fault notification, and etc. For example, SONETAIS/RDI (alarm indication signal/remote defect indication) protocols maybe used to notify terminating nodes of an optical circuit of the faultcondition in the network.

An optical network is a collection of optical network devicesinterconnected by links made up of optical fibers. Thus, an opticalnetwork is a network in which the physical layer technology isfiber-optic cable. Cable trunks are interconnected with opticalcross-connects (OXCs), and signals are added and dropped at opticaladd/drop multiplexers (OADMs). The optical network devices that allowtraffic to enter and/or exit the optical network are referred to asaccess nodes; in contrast, any optical network devices that do not arereferred to as pass-thru nodes (an optical network need not have anypass-thru nodes). Each optical link interconnects two optical networkdevices and typically includes an optical fiber to carry traffic in bothdirections. There may be multiple optical links between two opticalnetwork devices.

A given fiber can carry multiple communication channels simultaneouslythrough a technique called wavelength division multiplexing (WDM), whichis a form of frequency division multiplexing (FDM). When implementingWDM, each of multiple carrier wavelengths (or, equivalently, frequenciesor colors) is used to provide a communication channel. Thus, a singlefiber looks like multiple virtual fibers, with each virtual fibercarrying a different data stream. Each of these data streams may be asingle data stream, or may be a time division multiplex (TDM) datastream. Each of the wavelengths used for these channels is oftenreferred to as a lambda.

A lightpath is a one-way path in an optical network for which the lambdadoes not change. For a given lightpath, the optical nodes at which itspath begins and ends are respectively called the source node and thedestination node; the nodes (if any) on the lightpath in-between thesource and destination nodes are called intermediate nodes. An opticalcircuit is a bidirectional, end-to-end (between the access nodesproviding the ingress to and egress from the optical network for thetraffic carried by that optical circuit) path through the opticalnetwork. Each of the two directions of an optical circuit is made up ofone or more lightpaths. Specifically, when a given direction of theend-to-end path of an optical circuit will use a single wavelength, thena single end-to-end lightpath is provisioned for that direction (thesource and destination nodes of that lightpath are access nodes of theoptical network and are the same as the end nodes of the opticalcircuit). However, in the case where a single wavelength for a givendirection will not be used, wavelength conversion is necessary and twoor more lightpaths are provisioned for that direction of the end-to-endpath of the optical circuit. Thus, a lightpath comprises a lambda and apath (the series of optical nodes (and, of course, the interconnectinglinks) through which traffic is carried with that lambda).

FIGS. 1A and 1B are block diagrams illustrating an optical circuit of atypical SONET/SDH based optical network. In the SONET/SDH world, AIS/RDIsignals are generated by the first node that detects a failure of lossof signal (LOS) (e.g., a loss of an electrical signal) in order tosuppress the alarms. Both AIS and RDI may be used as triggers toinitiate a protection switch action. Note that because the SONET basednetwork assumes that the signal is fully regenerated at each node, onlyone node would ever detect a LOS on its ingress. On the egress of thatnode, it would still send a framed SONET signal that contained null datawith alarm information in its overhead. Downstream nodes would thus notdetect a LOS but would see AIS in the overhead. The downstream nodewould associate defects related to that signal to the fault reported byan upstream node.

Referring to FIG. 1A, where there is a unidirectional path failure, theintermediate node C is the first node to detect such a failure. Sinceeach of the nodes in the SONET/SDH based network regenerates signals atits respective egress. The downstream of the path (e.g., nodes D and E)still receive optical signals. Typically, in response to the detection,intermediate node C sends AIS signals to both downstream nodes to notifythe fault conditions. The terminating node (e.g., node E) may return anRDI signal to its upstream nodes (e.g., nodes A-D) of the opticalcircuit.

In a case of bi-directional path failures, as shown in FIG. 1B, bothnodes B and C send AIS signals to their respective downstream adjacentnodes (e.g., nodes D and A) for the notification purposes. Thedownstream nodes that receive such notification signals may rebroadcastthe notification messages (e.g., an AIS signal) to its respectiveadjacent downstream nodes.

As a result, each of the intermediate nodes may be required to receivesuch notification messages, convert the optical notification messagesinto electrical signals, and regenerate another notification message toits adjacent nodes.

Photonic switching equipment (e.g., equipment that does not typicallyperform optical to electrical conversion of switching, with exception ofadding and dropping traffic) used in all-optical networks, although notwidely deployed yet, it is typically based on the GMPLS architecture.The GMPLS architecture also uses signaling protocols, such as RSVP-TE,to perform hop-by-hop data path establishment, removing, and faultnotification. When a fault on a data path is detected, a faultnotification message is sent hop-by-hop to the source and destinationnodes. Such a notification relies on the transmission speed of thesignaling channel which is typically 10/100 Mbps.

Such notification messages (e.g., AIS/RDI or RSVP-TE) may be queuedduring the transmission (e.g., particularly, during the signalconversions between the electrical domain and the optical domain). Asthe optical network grows, particularly, in a mesh optical network, suchnotification messages are getting larger and larger which put a heavyburden on the network traffic. As a result, the fault notification maybe delayed significantly.

SUMMARY OF THE INVENTION

A method and apparatus for fault notification in an optical network aredescribed herein. In one embodiment, an exemplary process includesdetecting at a node that at least a portion of a first unidirectionalpath of an optical circuit is down, where the first unidirectional pathis originated from a first terminating node. In response to thedetection, the node signals the first terminating node by removing atleast a portion of light of a second unidirectional path in an oppositedirection of the first unidirectional path of the optical circuit, toindicate a path between the node and the first terminating node is down.Other features of the present invention will be apparent from theaccompanying drawings and from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings in which likereferences indicate similar elements.

FIGS. 1A and 1B are block diagrams illustrating a typical SONET/SDHbased fault notification of an optical network.

FIGS. 2A and 2B are block diagrams illustrating an example of awavelength division multiplex optical circuit according to oneembodiment of the invention.

FIGS. 3A and 3B are block diagrams illustrating an example of awavelength division multiplex optical circuit according to anotherembodiment of the invention.

FIGS. 4A and 4B are block diagrams illustrating an example of awavelength division multiplex optical circuit according to anotherembodiment of the invention.

FIG. 5A is a block diagram illustrating an example of an access node ofa wavelength division multiplex optical network according to oneembodiment of the invention.

FIG. 5B is a block diagram illustrating an example of an access node ofa wavelength division multiplex optical network according to analternative embodiment of the invention.

FIG. 6 is a flow diagram illustrating an example of a process for faultnotification in a wavelength division multiplex optical networkaccording to one embodiment of the invention.

DETAILED DESCRIPTION

A method and apparatus for fault notification in an optical network aredescribed herein. In one embodiment, instead of using a signal at eachnode receiving the fault notification that requires a conversion betweenan optical domain and an electrical domain (e.g., AIS/RDI or RSVP-TE) todetect a failure as in a conventional approach, the presence or absenceof light corresponding to a wavelength of a path (path/wavelength), alsoreferred to as a channel or a lambda, is used as an indication whetherthe respective path/wavelength is down or broken, to notify theterminating nodes of the path/wavelength of the optical circuit (e.g.,the source and the destination nodes). Thus, the notification does notrely on a particular type of electronic framing or packet scheme. As aresult, overall end-to-end path notification has been greatly improved.

In the following description, numerous specific details are set forth(e.g., such as logic resource partitioning sharing/duplicationimplementations, types and interrelationships of system components, andlogic partitioning/integration choices). However, it is understood thatembodiments of the invention may be practiced without these specificdetails. In other instances, well-known circuits, software instructionsequences, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to effect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

In the following description and claims, the terms “coupled” and“connected,” along with their derivatives, may be used. It should beunderstood that these terms are not intended as synonyms for each other.Rather, in particular embodiments, “connected” may be used to indicatethat two or more elements are in direct contact with each other (e.g.,physically, electrically, optically, etc.). “Coupled” may mean that twoor more elements are in direct contact (physically, electrically,optically, etc.). However, “coupled” may also mean that two or moreelements are not in direct contact with each other, but yet stillco-operate or interact with each other.

FIGS. 2A and 2B are block diagram illustrating an exemplary faultnotification in an all-optical network according to one embodiment ofthe invention. In contrast to the SONET/SDH based optical network,AIS/RDI mechanisms may not be applied in a straightforward fashion tooptically transparent networks. For example, if a fiber is cut, everydownstream node will detect loss of signal (LOS) (e.g., loss of opticalsignals or loss of light). According to one embodiment of the invention,only terminating nodes generate AIS or RDI type signals.

Referring to FIGS. 2A and 2B, according to one embodiment, the exemplaryoptical circuit includes a unidirectional path 201 from terminating nodeA to terminating node E and unidirectional path 202 from terminatingnode E to terminating node A. Between terminating nodes A and A, theremay be zero or more intermediate nodes, such as, for example,intermediate nodes B, C, and D. Paths 201 and 202 may be physicallyimplemented within a single fiber with opposite directions.Alternatively, paths 201 and 202 may include multiple optical fibers.Each of the paths 201 and 202 may include multiple wavelengths, alsoreferred to as channels or lambdas. For example, in a typical WDM(wavelength division multiplexing) network, each fiber may include up to40 wavelengths. Alternatively, there may be up to 80 wavelengths in aDWDM (dense WDM) network. It will be appreciated that more or lesswavelengths may be implemented within a fiber or a path. Terminatingnodes A and E may be a source or a destination node of the opticalcircuit.

Referring to FIG. 2A, where there is a unidirectional path failure,exemplary optical circuit 200, as an example, includes terminating nodesA and E (also referred to as end nodes) and zero or more intermediatenodes B, C, and D in between. The exemplary optical circuit 200 may be apart of an optical ring network or an optical mesh network. It will beappreciated that more, less, or no intermediate nodes may be implementedbetween the terminating nodes A and E. In one embodiment, exemplaryoptical circuit 200 includes a unidirectional path 201 from terminatingnode A to terminating node E and a unidirectional path 202 fromterminating node E to terminating node A. Paths 201 and 202 may beimplemented within a single fiber. Alternatively, paths 201 and 202 maybe implemented within different fibers. Terminating nodes A and E may bea source or a destination node of the optical circuit. The intermediatenodes B, C, and U may or may not be access nodes.

The unidirectional paths 201 and 202 may be established using a varietyof techniques apparent to those with ordinary skill in the art.According to one embodiment, the unidirectional paths 201 and 202 may beestablished and provisioned based on a service level topology associatedwith the respective path or paths. Further detailed informationconcerning establishments and provisioning of the paths may be found ina co-pending U.S. Patent Published Application No. 2004/0247317, filedJan. 9, 2004, entitled “A Method and Apparatus for a Network Database inan Optical Network”, and assigned to a common assignee of the presentapplication. The above-identified U.S. patent application is herebyincorporated by reference.

For illustration purposes, it is assumed that terminating node A is asource node of the optical circuit while terminating node E is adestination node of the optical circuit. In this example, path 201 maybe referred to as a transmission path while path 202 may be referred toas a return path corresponding to the transmission path. When a failureoccurs between the path from node B to node C, all downstream nodes(e.g., nodes C, D, and E) will detect LOS, where absence of signal isdepicted as a dotted line. The failure may be resulted from a fiberfailure, such as. for example, a fiber cut between nodes B and C.Alternatively, the failure may be resulted as a wavelength (e.g., achannel or a lambda) failure within the path, such as, for example, themalfunctioning laser device corresponding to that wavelength.

According to one embodiment, unlike the conventional approaches, whereeach intermediate node (e.g., nodes C and D) may be required to send anotification message (e.g., AIS/RDI or RSVP-TE protocols) to notify itsrespective adjacent nodes, only the terminating node E will respond tothe LOS and generate a notification signal via path 202 to notify theopposite terminating node A, while the intermediate nodes (e.g., nodesB, C, and D) do not send notification messages in response to thedetection of LOS.

In a case of bidirectional failure, as shown in FIG. 2B, the failuresbetween nodes B and C are detected by the terminating nodes A and Fbecause of LOS detected at the terminating nodes. Similar to the exampleof unidirectional failure illustrated in FIG. 2A, the intermediate nodesB, C, and D would not send any notification messages in response to thedetected LOS. As a result, the detection and notification of thefailures have been greatly improved.

FIGS. 3A and 3B are block diagram illustrating an example of a faultnotification in an optical circuit of an all-optical network accordingto another embodiment of the invention. In one embodiment, exemplaryoptical circuit 300 includes multiple access nodes interconnected viaone or more links. Each of the access nodes detects at a node, which mayor may not be an access node, that at least a portion of a firstunidirectional path of an optical circuit is down, the firstunidirectional path being originated from a first terminating node, andsignals the first terminating node by removing at least a portion oflight of a second unidirectional path in an opposite direction of thefirst unidirectional path of the optical circuit, to indicate a pathbetween the node and the first terminating node is down.

Referring to FIG. 3A, according to one embodiment, the exemplary opticalcircuit includes a unidirectional path 301 from terminating node A toterminating node E and unidirectional path 302 from terminating node Eto terminating node A. Between terminating nodes A and F, there may hezero or more intermediate nodes, such as, for example, intermediatenodes B, C, and D. Paths 301 and 302 may be physically implementedwithin a single fiber with opposite directions. Alternatively, paths 301and 302 may include multiple optical fibers. Each of the paths 301 and302 may include multiple wavelengths, also referred to as channels orlambdas. For example, in a typical WDM (wavelength divisionmultiplexing) network, each fiber may include up to 40 wavelengths.Alternatively, there may be up to 80 wavelengths in a DWDM (dense WDM)network. It wilt be appreciated that more or less wavelengths may beimplemented within a fiber or a path. Terminating nodes A and E may be asource or a destination node of the optical circuit. The intermediatenodes B, C, and D may or may not be access nodes.

The unidirectional paths 301 and 302 may be established using a varietyof techniques apparent to those with ordinary skill in the art.According to one embodiment, the unidirectional paths 301 and 302 may beestablished and provisioned based on a service level topology associatedthe respective path or paths, such as those illustrated in theabove-identified incorporated co-pending patent application.

For illustration purposes, it is assumed that terminating node A is asource node of the optical circuit while terminating node E is adestination node of the optical circuit. In this example, path 301 maybe referred to as a transmission path while path 302 may be referred toas a return path corresponding to the transmission path. When a failureoccurs between the path from node B to node C, all downstream nodes(e.g., nodes C, D, and E) will detect a LOS (e.g., loss of light), whereabsence of signal is depicted as a dotted line. The failure may beresulted from a fiber failure, such as, for example, a fiber cut betweennodes B and C. Alternatively, the failure may be resulted as awavelength (e.g., a channel or a lambda) failure within the path, suchas, for example, the malfunctioning laser device corresponding to thatwavelength.

According to one embodiment, unlike the conventional approaches, whereeach intermediate node (e.g., nodes C and D) may be required to send anotification message (e.g., AIS/RDI or RSVP-TE protocols) to notify itsrespective adjacent nodes, only the terminating node E will respond tothe LOS and remove at least a portion of the light of path 302 to notifythe opposite terminating node A, while the intermediate nodes (e.g.,nodes B, C, and D) do not send perform any notification in response tothe detection of LOS. Unlike the embodiments illustrated in FIGS. 2A and2B, the terminating node E, in this case, a destination node, turns offthe light of the corresponding return path (e.g., path 302 as depictedas dotted lines shown in FIG. 3B) as a signal to notify the oppositeterminating node A that at least a portion of path 301 or path 302 isdown. In one embodiment, the light may be turned off by turning off thecorresponding laser(s) or alternatively, by removing the correspondingphotonic cross connect(s) associated with the path(s). Terminating nodeA, in this case, a source node, may be notified by not receiving anoptical signal (e.g., light) of path 302. As a result, terminating nodeE does not have to convert the optical signal into an electrical signalto generate a notification signal, such as an AIS/RDI or RSVP-TE signal,to notify terminating node A.

FIGS. 4A and 4B are block diagrams illustrating an example of a faultnotification in an optical circuit of an all-optical network accordingto another embodiment of the invention. The exemplary optical circuitmay be viewed as a detailed aspect of the embodiments shown in FIGS. 3Aand 3B. According to one embodiment, in response to the detection of theLOS of a wavelength of a unidirectional path (e.g., path/wavelength),each of the downstream nodes checks whether the respective node is aterminating node of the optical circuit, such as, for example, adestination node of the optical circuit. If it is determined that therespective node is a terminating node of the optical circuit, theterminating node removes the light of a wavelength in an oppositedirection with respect to the failed wavelength. In one embodiment, thelight of the wavelength in the opposite direction may be removed byturning off the corresponding laser associated with the wavelength.Alternatively, the light may be removed by removing the correspondingphotonic cross connect associated with the wavelength. As a result, theother terminating node at the other end of the optical circuit (e.g.,the source node) will receive no light of that wavelength as anindication of a failed wavelength within the optical circuit.

Referring to FIG. 4A, in one embodiment, exemplary optical circuit 400includes terminating nodes 401 and 403, and zero or more intermediatenodes 402. The terminating nodes 401 and 403 may be a source node or adestination node of the optical circuit. Each of the nodes 401-403includes one or more photonic cross connects (PXCs) (e.g., PXCs 404 406)for switching traffic from the respective ingress port to an egressport. The nodes 401 403 of the optical circuit 400 are interconnectedvia one or more links. Each of the links may include one or more fibersand each fiber may include one or more wavelengths (also referred to aschannels or lambdas), such as, for example, 40 wavelengths in a WDMnetwork or 80 wavelengths in a DWDM network.

In this embodiment, for illustration purposes, it is assumed thatterminating node 401 is a source node of the optical circuit 400 whileterminating node 403 is a destination node of the optical circuit.Unidirectional path 420 from terminating node 401 to terminating node403 may include one or more fibers 415 and 416. Unidirectional path 430from terminating node 403 to terminating node 401 may include one ormore fibers 417 and 418. Fibers 415-418 may be the same fiber havingopposite unidirectional paths 420 and 430. Alternatively, fibers 415-418may be different fibers. The one or more intermediate nodes 402 may ormay not be access nodes.

In this example, path 420 may be referred to as a transmission pathwhile path 430 may be referred to as a return path corresponding to thetransmission path. When a failure of a wavelength of path 420 occursbetween terminating node 401 and intermediate node 402, such as, forexample, a wavelength of fiber 415 of path 420 is down as illustrated inthe balloon), all downstream nodes (e.g., terminating node 403 and anyof the intermediate nodes between the failure and that terminating node)will detect a LOS (e.g., loss of light), where absence of signal isdepicted as a dotted line. The failure may be resulted from a wavelength(e.g., a channel or a lambda) failure within the path, such as, forexample, the malfunctioning laser device corresponding to thatwavelength.

In this example, the failed wavelength of path 420 (e.g.,path/wavelength) is represented by wavelengths 407 to 410 across everynodes of the path 420. As a result of a failed wavelength of fiber 415,downstream nodes 403 and 402 will detect the LOS on wavelengths 408-410,as depicted as dotted lines. According to one embodiment, in response tothe detection of LOS, each of the downstream nodes (e.g., terminatingnode 403 and zero or more intermediate nodes 402) checks whether therespective node is a terminating node of the optical circuit. If so, theterminating node will turn off the light of the corresponding wavelengthof the unidirectional return path of the optical circuit.

In this example, since the one or more intermediate nodes 402 are notthe terminating nodes of the optical circuit 400, the one or moreintermediate nodes 402 will not performing signaling the terminatingnodes of the optical circuit. However, when terminating node 403 detectsthe LOS, the terminating node 403 turns off the light of thecorresponding wavelength (represented by wavelengths 411-414) of theunidirectional return path 430, as illustrated in FIG. 4B. As a result,terminating node 401 receives no light on the corresponding wavelength414 of path 430. Since no notification messages or protocols areinvolved in this embodiment, there is no need to convert the opticalsignals to electrical signals in order to notify a terminating nodeconcerning the failures of a wavelength of a path. As a result, aterminating node is notified in a much quicker manner. In oneembodiment, the light of the wavelength in the opposite direction may beremoved by turning off the corresponding laser associated with thewavelength. Alternatively, the light may be removed by removing thecorresponding photonic cross connect associated with the wavelength.Other methods for removing at least a portion of light may be utilizedwithin the scope of the embodiments of the invention.

FIG. 5A is a block diagram illustrating an example of an access node ofan all-optical network according to one embodiment of the invention. Inone embodiment, exemplary access node 500 includes, but not limited to,a detection module to detect that at least a portion of a firstunidirectional path of an optical circuit is down, the firstunidirectional path being originated from a first terminating node, anda control module coupled to the detection module to signal the firstterminating node by terminating at least a portion of light of a secondunidirectional path in an opposite direction of the first unidirectionalpath of the optical circuit, to indicate that the first unidirectionalpath is down.

Referring to FIG. 5A, according to one embodiment, exemplary access node500 includes one or more photonic cross connects (PXCs) 504 and 505 toprovide cross connect services for the unidirectional or bi-directionaltraffic. Each of the PXCs includes one or more individual PXCs (e.g.,PXCs 506-509) to handle cross connect services for each wavelength(e.g., lambda or channel). In addition, according to one embodiment,exemplary access node includes a control module 501 coupled to the PXCs504 and 505, and one or more photonic detectors 502 and 503. The PXCs504 and 505 may be the same PXCs for handling bi-directional traffic. Inaddition, the exemplary access node 500 includes one or more add/dropmultiplexers (ADMs) (not shown) to allow traffic get on or off therespective optical circuit.

According to one embodiment, photonic detectors 502 and 503 may be usedto detect whether there is a LOS on a wavelength. The detection may beperformed on a wavelength basis. In one embodiment, the photonicdetectors 502 and 503 may be photo diodes that can detect presence orabsence of the light on a per wavelength basis. When a LOS of awavelength is detected by the photonic detector, the photonic detectornotifies the control module. According to one embodiment, the controlmodule determines whether the access node is a terminating node of anoptical circuit. If the access node is determined to be a terminatingnode of an optical circuit, the control module instructs a PXC to turnoff a light of a wavelength in an opposite direction of the failedwavelength to notify the other terminating node of the optical circuitfailure. Alternatively, the control module may alter the PXC or switchoff the laser, etc. for the purposes of signaling.

For example, for illustration purposes, it is assumed that wavelength510 is down or broken (e.g., a loss of light) as illustrated by a dottedline. As a result, a LOS detected by photonic detector 502. In responseto the detection, photonic detector 502 notifies control module 501regarding the status of wavelength 510. Control module 501 determineswhether access node 500 is a terminating node of the optical circuitcorresponding to the path/wavelength 510. If control module 501determines that access node 500 is a terminating node of the opticalcircuit, control module 501 instructs PXCs 505 to turn off the light ofthe corresponding wavelength of a path in an opposite direction of thepaths controlled by PXCs 504, as illustrated by a dotted line of theballoon of path 511. If control module 501 determines that access node500 is not a terminating node of the optical circuit, control module 501may simply ignore the detection with respect to the signaling theterminating nodes of the failure.

Similarly, photonic detector 503 may detect any wavelength of path 512that has lost light. In return, photonic detector 503 may notify controlmodule 501 regarding the LOS of the wavelength of path 512. If theaccess node 500 is a terminating node of the corresponding opticalcircuit, control module 501 may instruct the PXCs 504 to turn off thelight of the corresponding wavelength in the opposite direction of path512, such as, for example, a wavelength in at least one of paths 513 and514, to notify the other terminating node of the corresponding opticalcircuit. In this manner, there is no need to convert the optical signalsinto electrical signals and use the notification packets (e.g., AIS/RDIor RSVP-TE) to notify the other terminating node (e.g., a source node).As a result, the speed of fault notification has been greatly improved.In one embodiment, the light of the wavelength may be removed by turningoff the corresponding laser associated with the wavelength.Alternatively, the light may be removed by removing the correspondingphotonic cross connect associated with the wavelength.

Alternatively, according to another embodiment of the invention, thephotonic detector may detect the loss of optical signals (e.g., loss oflight) across the connections between the PXCs and the ADM, as shown inFIG. 5B. In this embodiment, the optical signals that are detected bythe photonic detector are the optical signals terminated or originatedat a terminating node of an optical circuit. That is, the opticalsignals of a non-terminating node would not be detected by the photonicdetector because the optical signals are not terminated or originated ata non-terminating node. Therefore, when the phonotic detector, which islocated between the PXC and the ADM, detects a loss of light at one ormore path/wavelengths, the corresponding node would most likely aterminating node of the respective optical circuit. As a result, thenode may not need to determine whether the corresponding node is aterminating node before responding the detection of the loss of light,because when the control module receives such a detection from thephotonic detector, the corresponding node would most likely be aterminating node of the optical circuit. It will be appreciated thatother configurations may exist.

Further detailed information concerning the exemplary nodes illustratedin FIGS. 5A and 5B can be found in issued U.S. Pat. No. 7,174,066, byChristopher M. Look, having an applicant's reference number of 8433P010,entitled “A Method And An Apparatus To Detect Signal Failure On A PerWavelength Basis”, filed Feb. 23, 2004, and assigned to a commonassignee of the present application, which is hereby incorporated byreference.

FIG. 6 is a flow diagram illustrating an example of a process for faultnotifications in an optical network according to one embodiment of theinvention. Exemplary process 600 may be performed by a processing logicthat may comprise hardware (circuitry, dedicated logic, etc.), software(such as is run on a dedicated machine), or a combination of both. Inone embodiment, exemplary process 600 includes, but not limited to,detecting at a node that at least a portion of a first unidirectionalpath of an optical circuit is down, the first unidirectional path beingoriginated from a first terminating node, and signaling the firstterminating node by removing at least a portion of light of a secondunidirectional path in an opposite direction of the first unidirectionalpath of the optical circuit, to indicate a path between the node and thefirst terminating node is down.

Referring to FIG. 6, at block 601, a node detects that at least aportion of a first unidirectional path of an optical circuit is down,where the first unidirectional path is originated from a firstterminating node of the optical circuit, such as, for example, a sourcenode. In one embodiment, the detection may be performed based on a perwavelength basis. For example, a node may detect that one or morewavelengths of the path (e.g., path/wavelength) are down. According toone embodiment, the node may detect the failed wavelength by detectingthe loss of light of the respective wavelength as an indication of thefailed wavelength.

In response to the detection, at block 602, the node determines whetherthe respective node is a terminating node of the optical circuit, suchas, for example, a destination node of the optical circuit. In oneembodiment, the determination is performed based on information relatedto the path containing the failed wavelength, which may be stored in adatabase maintained by the node. In the embodiment as shown in FIG. 5B,the determination of whether the node is a terminating node may notneeded.

If the node is determined to be a terminating node of the opticalcircuit, at block 603, the node signals the first terminating node via asecond wavelength of a path in an opposite direction of the failed pathof the optical circuit to indicate the first path/wavelength is down. Inone embodiment, the node turns off the light of the secondpath/wavelength, such that the first terminating node (e.g., the sourcenode) would not receive the light of the second path/wavelength as anindication of the failure of the first path/wavelength. In oneembodiment, the light of the second path/wavelength may be removed byturning off the corresponding laser associated with the secondpath/wavelength. Alternatively, the light may be removed by removing thecorresponding photonic cross connect associated with the secondpath/wavelength. If the node is not a terminating node of the opticalcircuit (e.g., a destination node of the optical circuit), the node maysimply ignore the detection of LOS of the first path/wavelength for thepurposes of signaling the terminating nodes of the failure. Otheroperations apparent to those with ordinary skill in the art may beincluded.

Thus, A method and apparatus for fault notifications in an opticalnetwork have been described herein. In the foregoing specification, theinvention has been described with reference to specific exemplaryembodiments thereof. It will be evident that various modifications maybe made thereto without departing from the broader spirit and scope ofthe invention as set forth in the following claims. The specificationand drawings are, accordingly, to be regarded in an illustrative senserather than a restrictive sense.

1. A method performed by a node of a wavelength multiplex opticalnetwork, the method comprising: detecting at the node that at least aportion of functionality of a wavelength of a first unidirectional path(first path/wavelength) of an optical circuit fails to operate, thefirst unidirectional path being originated from a first terminating nodeas a source node for reaching a second terminating node as a destinationnode of the first unidirectional path, the first terminating node andthe second terminating node forming the optical circuit, wherein theoptical circuit includes a nonterminating node; determining within thenode whether the node is the second terminating node of the opticalcircuit in response to detecting a failure of the first unidirectionalpath; if it is determined that the node is the second terminating nodeof the optical circuit, the node signaling the first terminating node byremoving a light of a second wavelength of a second unidirectional path(second path/wavelength) in an opposite direction of the firstunidirectional path of the optical circuit, to indicate the failure ofthe first path/wavelength; and the node ignoring the failure of thefirst unidirectional path without removing the light of the secondwavelength of the second unidirectional path if it is determined thatthe node is not the second terminating node of the optical circuit. 2.The method of claim 1, wherein the first terminating node is notified ofthe detection by not receiving at least a portion of the light of thesecond unidirectional path.
 3. The method of claim 1, wherein the firstunidirectional path is detected based on a loss el at least a portion oflight of die first unidirectional path.
 4. The method of claim 1,further comprising: detecting a wavelength of the first unidirectionalpath (first path/wavelength) is down; and signaling the firstterminating node via a second path/wavelength of the secondunidirectional path with respect to the status of the firstpath/wavelength.
 5. The method of claim 4, wherein the firstpath/wavelength is detected based on a less of light of the firstpath/wavelength, and wherein the first terminating node is notified bynot receiving the light of the second path/wavelength.
 6. The method ofclaim 1, wherein the first and second unidirectional paths are withindifferent fibers.
 7. The method of claim 1, wherein the signaling isperformed without converting optical signals of the first unidirectionalpath to electrical signals specifically used to signal the firstterminating node that the path between the node and the firstterminating node is down.
 8. An apparatus, comprising: a node to becoupled to a wavelength division multiplex optical network, the nodeincluding, a detection module to detect that a wavelength of a firstunidirectional path (first path/wavelength) of an optical circuit failsto perform, the first unidirectional path being originated from a firstterminating node as a source node for reaching a second terminating nodeas a destination node of the first unidirectional path, the firstterminating node and the second terminating node forming the opticalcircuit, wherein the optical circuit includes a non-terminating node,and a control module coupled to the detection module to determinewhether the node is the second terminating node in response to detectinga failure of the first unidirectional path, and if it is determined thatthe node is the second terminating node, the control module isconfigured to signal the first terminating node by removing a light of asecond wavelength of a second unidirectional path (secondpath/wavelength) in an opposite direction of the first unidirectionalpath of the optical circuit, to indicate the first path/wavelength isdown, wherein the control module is configured to ignore failure of thefirst unidirectional path without removing the light of the secondwavelength of the second unidirectional path if it is determined thatthe node is not the second terminating node.
 9. The apparatus of claim8, wherein the first terminating node is notified of the detection bynot receiving at least a portion of light of the second unidirectionalpath.
 10. The apparatus of claim 8, wherein the first unidirectionalpath is detected based on a loss of at least a portion of light or thefirst unidirectional path.
 11. The apparatus of claim 8, wherein thedetecting nodule detects a wavelength of the first unidirectional path(first path/wavelength) is down, and wherein the control module signalsthe first terminating node via a second wavelength of the secondunidirectional path (second path/wavelength) with respect to the statusof the first path/wavelength.
 12. The apparatus of claim 11, wherein thefirst path/wavelength is detected based on a loss of light of the firstpath/wavelength, and wherein the first terminating node is notified bynot receiving the light of the second path/wavelength.
 13. The apparatusof claim 8, wherein the first and second unidirectional paths are withindifferent fibers.
 14. The apparatus of claim 8, wherein the detectionmodule signals the first terminating node without converting therespective optical signals of the first unidirectional path toelectrical signals specifically used to signal the first terminatingnode that the path between the node and the first terminating node isdown.
 15. A wavelength multiplex optical network, comprising: aplurality of nodes interconnected via one or more links, each of theplurality of nodes to detect that at least a portion of functionality ofa wavelength of a first unidirectional path (first path/wavelength) ofan optical circuit fails to operate, the first unidirectional path beingoriginated from a first terminating node as a source node for reaching asecond terminating node as a destination node of the firstunidirectional path, the first terminating node and the secondterminating node forming the optical circuit, wherein the opticalcircuit includes a non-terminating node; determine whether the node isthe second terminating node of the optical circuit in response todetecting a failure of the first unidirectional path; if it isdetermined that the node is the second terminating node of the opticalcircuit, signal the first terminating node by removing a light of thesecond wavelength of a second unidirectional path (secondpath/wavelength) in an opposite direction of the first unidirectionalpath of the optical circuit, to indicate the failure of the firstpath/wavelength; and ignore the failure of the first unidirectional pathwithout removing the light of the second wavelength of the secondunidirectional path if it is determined that the node is not the secondterminating node the optical circuit.